1. The Field of the Invention
The present invention relates to analog circuits, and more specifically to current measuring circuits and transconductance amplifiers that enable current measurement.
2. Background and Related Art
Analog circuits have revolutionized the way people work and play and have contributed enormously to the advancement of humankind. Analog circuits may perform quite complex functions by uniquely combining smaller analog circuits that perform more fundamental functions. One common analog clement is a current measurement circuit that measures the current at a node in a circuit or passing through a particular load. The current measurement circuit may be on-chip with the load whose current it is measuring, or may be separate and apart from the load.
There are many conventional current measurement circuits. A large portion of such current measurement circuits (including many current meters) use a resistor in series with the load. Such conventional current measurement circuits calculate the current flow by measuring the voltage drop across the series combination of the load and the resistor.
FIG. 6 illustrates one conventional improvement to this current measurement circuit in the form of circuit 600. The circuit 600 improves measurement accuracy by compensating for the inclusion of the series resistance in measurement accuracy. However, the circuit operates with best accuracy when the load is driven by a low impedance node. The circuit will not operate accurately when the input node Vin is a high impedance node. Furthermore, the circuit 600 also limits the voltage headroom of the amplifier 601 as the load voltage VL approaches the level of voltage supply. Such loss in headroom is disadvantageous because it limits the voltage range VL over which the current can be measured.
Other conventional current measurement circuits copy the current through a current carrying transistor as shown in the current measurement circuit 700 of FIG. 7. The circuit 700 allows current to flow in only one direction. However, many load circuits may be designed to allow current flow bi-directionally. In order to properly evaluate such loads, it would be advantageous to measure current flowing bi-directionally.
Accordingly, what would be advantageous is a current measurement circuit that works with high impedance nodes, does not result in a loss of amplifier headroom, works for bi-directional current, and reduces load voltage variations due to load current.